1.Technical Field
The present invention relates generally to separating liquids from solid particles having magnetic properties. Specifically, the system and method may be used to separate liquid from solid catalyst particles and may be applied in multi-phase catalytic reactors where the catalyst comprises solids with magnetic properties. Such multiphase catalytic reactors may be Fischer-Tropsch (FT) reactors of a Fischer-Tropsch synthesis process.
2. Background of the Invention
Several methods for separating liquids and solids in a Fischer-Tropsch process/reactor system have been proposed. These methods include settling, filtration, and combinations thereof. Magnetic separation alone has also been proposed. Typically, primary separation and secondary separation are utilized, with primary separation removing the larger solids and secondary separation removing smaller solids. Both the primary separators and the secondary separators may be settlers. Primary settlers may be dynamic settlers. In certain applications, primary separators are cross-flow filtration units. Secondary separators are conventionally cross-flow filtration devices, or settlers.
Settling is a method utilized to separate solids and liquids, and may be applied in Fischer-Tropsch processes/reactor systems. Settlers may be of the vertical type or may be inclined settlers. See, for example, U.S. Pat. Nos. 6,068,760; 6,712,982; and 7,078,439. Inclined settlers, also known as lamella type settlers, may permit higher liquid removal rates than the same size vertical settler. The design of such settlers is based on particle settling velocity which is highly dependent on particle diameter. Thus, once a settler is designed, settling of particles of a particular diameter or larger is obtained. If attrition, etc., reduces the size of the particles, these particles may exit the settler with the liquid, thus contaminating the liquid. In a Fischer-Tropsch process, when catalyst particles exit the reactor, the particles not only contaminate the liquid product but also decrease the catalyst inventory in the reactor. Both of these events are detrimental to the process economics.
Fischer-Tropsch catalysts, which are typically either iron-based or cobalt-based, are prone to attrition. Typical particles of fresh catalyst have a size in the range of 20-100 microns. Attrition may result in the formation of particles having a size of less than 20 microns; in certain applications, particle size may even reach sub-micron levels. These smaller particles tend to plug filter media and/or alter the characteristics of the cake on the filter media, thus compacting the filter, which may become substantially impermeable. Filtration across compact cakes mandates a higher pressure drop across the filtration media to obtain the same amount of liquid filtrate. This creates a vicious cycle of higher pressure drop leading to more compact cakes and/or media plugging which may ultimately render the system ineffective.
Cross flow filtration is one of the most widely used methods of separation. Cross flow filtration is described, for example, in U.S. Pat. Nos. 6,929,754 and 6,833,078. In some applications, a “mild” cross flow filtration method is utilized. By this method, a ‘cake’ of catalyst particles is formed on the surface of the filter media, and this cake acts as the primary barrier for the prevention of solids passing through the filter media and contaminating the liquid. Some disadvantages of this method, however, are that the filter medium is usually prone to plugging by small particles which may be present due to physical and/or chemical attrition during the use of the media. Filter media are design for a certain micron rating. For example, with a micron rating of 20 microns, particles larger than 20 microns will theoretically be retained on the surface of the media. Particles smaller than 20 microns may travel through the media and exit or may get stuck within the pores of the filter medium due to agglomeration, shape and/or other factors. Even though a backwash method may be used to attempt to unplug the medium, the medium may become ineffective with time on stream. Eventually, the filter elements must be removed from the system and replaced.
Smaller particles, say less than 20 microns, and mainly those less than 10 microns and perhaps less than 1 micron tend to render a “mild” cross flow filtration process ineffective for separation of liquids and solids in Fischer-Tropsch processes. These smaller particles also cause separation of the particles from the liquid by sedimentation alone very difficult. The settling equipment tends to become large and thus economically impractical.
Magnetic separation alone has previously been proposed to separate solids and liquids in Fischer-Tropsch processes/reactors systems. For example, see “Magnetic Separation of Iron Catalysts from Fischer-Tropsch Wax,” R. R. Oder, Proceedings of the Petroleum Chemistry Division, ACS Annual Meeting, CA (Mar. 28-Apr. 1, 2004); and “Separation of Iron Catalysts from Fischer-Tropsch Wax,” R. R. Oder et al., Twentieth Annual Pittsburgh Coal Conference: Coal, Energy and the Environment, Pittsburgh, Pa. (Sep. 15-19, 2003). This form of separation comprises passing a slurry containing the liquid and solids through a vessel the walls of which have been magnetized. If the solid particles have magnetic properties, the particles tend to accumulate on the walls of the vessel and fall to the bottom of the vessel, continuing to travel in the direction of the slurry. Thus, particle-reduced liquid may be withdrawn from the top of the vessel. However, this method tends to be more effective for smaller particles, for example, sub-micron-sized particles. In order for the method to be effective for a broad range of particle sizes, for example, for particles having sizes from sub-micron to 100 microns, the equipment may have to be rather large and the power needed for the magnetization much higher than the power required for the separation of particles within a smaller size range.
In a Fischer-Tropsch process, wax product streams from which particles have been removed by primary and optionally secondary separation, are sent for product upgrading, PU. Catalyst-containing streams separated in primary and/or secondary separation may be recycled to the Fischer-Tropsch reactor or disposed according to regulations. Product upgrading processes at the back end of Fischer-Tropsch plants typically comprise hydrogenation, hydrocracking and/or isomerization processes, whereby the Fischer-Tropsch liquids produced in the Fischer-Tropsch reactors are refined to obtain desirable products. These product upgrading processes are often stringent in the amount of solids that can be tolerated in the liquid feed to be treated, usually limiting the solids content of the liquid feed to less than 10 ppm by weight. Particle reduction to the desired specification in the Fischer-Tropsch liquid product may be challenging.
Accordingly, there is a need in industry for reliable and efficient systems and methods for separating catalyst particles having magnetic properties from liquids.